1. Technical Field
The present disclosure relates to an optical method for determining several morphological parameters and physiological properties (hereafter abbreviated MP&PPs) of tissue. In particular, the present disclosure is directed to a method to determine MP&PPs of malignant as well as benign tissue lesions.
2. Description of the Related Art
Malignant melanoma is one of the most rapidly increasing cancers in the world. In the United States alone, the estimated incidence for 2008 is 62,480, which leads to an estimated total of 8,420 deaths per year. Successful treatment of melanoma depends on early detection by clinicians with subsequent surgical removal of tumors. Visual detection has its limitations, even when augmented with dermoscopy, especially with less experienced users. Attempts have thus been made to develop automated devices to assist in the screening of pigmented skin lesions for likelihood of melanoma. Several of these devices have digitalized dermoscopy-related features analyzed by artificial neural networks or support vector machine learning systems.
The optical properties of human skin in the ultraviolet spectral region have been studied for almost one hundred years [Hasselbalch, 1911; Everett et al., 1966], and non-invasive optical methods have been applied to study the physiological state of human skin for at least twenty years [Diffey et al., 1984]. A well-known application is blood-oxymetry, by which a relative blood-oxygenation index can be determined non-invasively from the scattered reflectance or transmittance of light at red and near-infrared (NIR) wavelengths [see e.g. Yaroslavsky et al., 2002]. The reflectance in that spectral region has also been used to determine other physiological properties. For example, the gradient of the reflectance spectrum between 620 nm and 720 nm depends on the total melanin content of the skin [Kollias and Baqer, 1985]. However, variations in the blood concentration, the thicknesses of the skin layers, and the scattering phase function of skin tissue, also affect the reflectance spectrum, and thereby the accuracy of the determination of blood oxygenation and total melanin content. Therefore, it is essential to perform a simultaneous determination of all optically important MP&PPs.
In order to determine tissue optical properties (as opposed to MP&PPs) from spectral reflectance measurements several different inversion schemes have been used, including partial least squares regression [Berger et al., 2000], neural networks [Kienle et al., 1996], fuzzy logic [Dam et al., 1998], and genetic algorithms [Zhang et al., 2005]. In contrast, this invention makes use of a nonlinear inversion scheme based on e.g. optimal estimation theory [Tikhonov, 1977; Twomey, 1977; Tarantola, 1987; Rodgers, 2000], combined with bio-optical models [which provide a link between MP&PPs and inherent optical properties (IOPs)] and accurate radiative-transfer modeling in coupled air-tissue systems. Also, this invention provides a method for deriving a set of additional morphological parameters (MPs) of tissue from reflectance measurements.